The present invention generally relates to a semiconductor diffusion type force sensing apparatus, and in particular to a semiconductor diffusion type force sensing apparatus which may be suitably used as a force sensing sensor built in robots and two- or three-dimensional input devices which serve as man-machine interfaces.
A conventional force sensing apparatus includes a strain producing body which is elastically deformed when an external force is applied thereto, and a plurality of sensing elements formed on the body and having an electric resistance which changes in accordance with a deformation of the body. This apparatus takes out a change of the electric resistance as an electric signal of those sensing elements to sense the applied external force.
A semiconductor diffusion type force sensing apparatus is one of the typical force sensing apparatuses. For example, the Japanese Laid-Open Application No. 221288/1985 has disclosed a semiconductor diffusion type force sensing apparatus, in which a plurality of sensing elements are formed on a flat plane of a strain producing substrate formed by a single crystal of silicon by a semiconductor planar process. The strain producing substrate is a flat plate ring having a regular octagon shape which is made of a single crystal of silicon. An external force is exerted on a surface of the strain producing substrate, and a two-dimensional distribution of three components Fx, Fy and Fz of the external force can be detected in accordance with change of the resistances of the sensing elements in response to the external force.
However, a procedure of cutting out the flat plate ring having the shape of a regular octagon from the silicon single crystal must be performed by using a combination of a dicing cut process, a laser process, an etching process and so on. Hence, the cutting-out procedure is very complex.
In order to overcome the above disadvantage of the conventional art, a semiconductor diffusion type force sensing apparatus has been proposed in the Japanese Patent Application No. 259049/1986 filed on Oct. 30, 1986 entitled "A FORCE DETECTING APPARATUS", the assignee of which is the same as the assignee of the present invention. According to this application, a plurality of sensing elements are obtained by forming p-type impurity diffused regions in an n-type silicon single crystal having the {110} plane. The sensing elements are aligned in X and Y axial directions which are orthogonal to each other. The X and Y axial directions have an angle of 45.degree. with respect to the [110] axis. The aligned sensing elements are mutually connected to form a bridge circuit.
However, the above force sensing apparatus cannot detect, with a high accuracy, a direction and a magnitude of the applied external force. This is because the direction of a maximum current flow through the sensing element does not coincide with the direction in which an extremal value of the piezoresistance coefficient is obtained.
Still another conventional semiconductor diffusion type force sensing apparatus has been proposed in the Japanese Laid-Open Patent Application No. 153173/1985. The disclosed force sensing apparatus utilizes the {111} plane of a p-type silicon region formed in an n-type silicon substrate (wafer) having a ring shape. That is, a plurality of sensing elements are provided by the p-type silicon {111} plane portions formed on a surface of the ring-shaped substrate. An external force is applied to a surface perpendicular to the surface having the sensing elements. The sensitivity with respect to the p-type silicon {111} plane does not depend on a crystal orientation thereof. Therefore, dispersion in the sensitivity among the sensing elements can be reduced.
However, it is very difficult to produce the ring-shaped semiconductor substrate. Additionally, as described in the above-identified laid-open application, the sensing elements can be suitably obtained by only the p-type silicon {111} plane. In other words, the n-type silicon {001} plane, n-type silicon {011} plane, p-type silicon {001} plane, p-type silicon {211} plane and the like are not suitable for forming the sensing elements. Moreover, the silicon substrate is designed so that an external force is exerted on a flat plane of the ring-shaped silicon substrate which is perpendicular to the sensing plane on which the sensing elements are arranged. For this reason, it is difficult to support the ring-shaped silicon substrate without affecting a deformation thereof due to the external force.